Major depressive disorder (MDD) is a chronic and life-threatening illness, afflicting over 10% of the adult population in the United States. However nearly 40% of MDD patients are treatment resistant. The lack of universally effective antidepressant treatments may be due to our incomplete understanding of the brain regions responsible for mediating depression. Studies with preclinical animal models of depression and brain imaging of human patients have implicated the mesocorticolimbic ventral tegmental area (VTA) reward circuitry, which has been understudied in depression. Recent studies have implicated VTA hyperactivity in mediating susceptibility to depression-related behaviors in chronic social defeat stress, a well-established animal model of depression. Further research is required to fully characterize the mechanisms by which stress induces pathological alterations in the VTA at the level of neural circuitry. Our preliminary studies focusing on the lateral hypothalamus (LH) input to the VTA (LH-VTA) indicate that LH-VTA hyperactivity mediates susceptibility. Mimicking the increase in firing activity of LH-VTA neurons by the use of photostimulation with optogenetics, an approach that can increase neuronal activity through light-sensitive channels in behaving animals, induces depression-like behaviors. In addition, our preliminary work indicates that LH-released hypocretin (orexin), a neuropeptide known to regulate reward, arousal and wakefulness, may mediate the VTA neuron responses to social defeat stress. In this proposal, I will use in vivo and in vitro electrophysiology and optogenetic approaches to investigate whether there are pathophysiological alterations in VTA neuron subpopulations induced by LH-VTA photostimulation, as well as how hypocretin regulates VTA neuron subpopulations in social defeat stress. These studies will provide insight into the role of LH-VTA neural circuitry in depression and reveal novel targets for antidepressant development.